1. Technical Field
This invention relates to a method for processing scrap tires and other discarded rubber items to obtain useful carbon products therefrom.
More specifically, this invention relates to a method for pyrolyzing scrap tires and other discarded rubber items to obtain a char, and to the further processing of that char to obtain carbon products suitable for use in a wide variety of industrial applications.
2. Description of Related Art
The pyrolytic conversion of scrap tires to obtain char and liquid and gaseous decomposition products is well known in the art. Such processes promise a useful and environmentally friendly solution to the disposal of a significant portion of the estimated 700 million scrap tires generated worldwide each year. However, to be economically successful, the process would have to be primarily supported through the sale of carbon based products in commodity markets. That has proven to be a significant barrier.
Pyrolytic char particles usually display a very wide size range, from less than one micron to more than one millimeter. The principal difficulty experienced in processing such char to obtain commercially acceptable products has been to obtain a very finely divided material of narrow particle size range having properties useful in rubber, elastomers, inks, pigments and plastics.
Attempts to obtain a commercially acceptable carbon product are well documented in the prior art. One such approach is described in U.S. Pat. No. 3,644,131 to Gotshall. The Gotshall patent asserted that a carbon product equivalent in quality to high grade carbon blacks can be obtained by retorting scrap tires and then comminuting the resulting char using a fluid energy mill to obtain a product having an average particle size of less than about 2.5 microns. The fluid energy mill used by Gotshall operated on steam at a temperature of about 450° F. at the nozzle exit. His mill was arranged with opposed nozzles to cause the carbon particles carried in a first stream to impinge at sonic velocity upon carbon particles carried in a second stream causing autogenous grinding of the colliding particles. The finely divided carbon product was then coated with a portion of the heavy oils from the retort to obtain a stable product.
Fader, in U.S. Pat. No. 5,037,628, disclosed a process that is generally similar to that of Gotshall in that tires were pyrolyzed in a retort to obtain a char, and a finely divided carbon product was produced from that char. Fader found that his char consisted of agglomerations, or clusters, of finer carbon particles mixed with unitary grit like particles. A carbon product, asserted to be comparable to commercial grade carbon blacks, was obtained by agitating the char to selectively de-agglomerate the carbon clusters without affecting the unitary particles which were thereafter separated from the smaller carbon particles. An air swept hammer mill, modified to provide only agitation, was used to de-agglomerate the carbon clusters. Data presented in the patent showed that the char material after de-agglomeration displayed a particle size range in which 90% of the particles were less than 150 microns in size.
Yet another approach for obtaining useful products from scrap tires and other waste rubber is described in U.S. Pat. No. 5,506,274 to Brown. The Brown patent states that particles of carbon black encapsulated in spheres of rubber are produced by passing small particles of waste rubber containing carbon black through a rotating or oscillating retort that is operated at a preferred temperature range of 500° F. to 850° F. Brown further teaches that dwell time of the waste rubber in passing from the entrance end to the exit end of the retort is critical to the proper formation of his desired spherical rubberized carbon black, and that the dwell time should be approximately 35-40 seconds.
The problems experienced with existing approaches to the pyrolytic processing of scrap tires have served to limit their usefulness and success. There remains a high degree of uncertainty regarding the economic feasibility of pyrolysis for recycling scrap tires. In fact, in May 2002, the Rubber Manufacturers Association stated that since 1985, a total of 75 pyrolysis projects have been announced, patented, licensed and promoted. Of those 75 projects, four were actually built and, as of May 2002, there were no commercially operational pyrolytic facilities in the United States, Canada, Japan, or Europe.
The large capital costs and high operating expenses of a fluid energy mill as was proposed by Gotshall makes that approach commercially impractical. Also, the grinding of char is a very demanding process. It requires production of a very fine and abrasive product at rates of one ton or more per hour on a continuous, day in and day out basis using simple, fool-proof equipment in order to be economically viable. The low yield of fine material obtained by de-agglomeration in the manner taught by Fader et al diminishes the attractiveness of that approach. It is very difficult, if not impossible, to dry-sieve carbon black because of its electrostatic properties as would be required were the Brown process to produce carbon black-like products rather than rubberized spheroids.
As may be readily appreciated, a process which alleviates and overcomes the technical deficiencies inherent in present practices and which produces high-value, marketable products would be a significant advance in the art.